s390/pci: Documentation for zPCI

s390/pci: Documentation for zPCI

BugLink: https://bugs.launchpad.net/bugs/1874056
There are changes in the usage of PCI for the user:
 - new kernel parameter
 - modification of the way functions are enumerated

Let's document these.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
(backported from commit de267a7c71ba6be7857da0185871759067513d9c)
Signed-off-by: Frank Heimes <frank.heimes@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>